Current mirror-A constant current circuit

A simple way to explain a current mirror as a constant current circuit.

1. CURRENT MIRROR-A
CONSTANT CURRENT
CIRCUIT
Prepared by: Dr. Kapil Gupta
Associate Professor, MMDU
Mullana(Ambala)

2. Basic Principle of “CURRENT MIRROR”
•Thetwotransistorimplementationofthecurrentmirroris
basedonthefundamentalrelationshipthat,twoequalsize
transistorsatthesametemperaturewiththesameV
BEhave
thesamecollectorcurrent.
•Acurrentmirrorisa
circuitblockwhichfunctions
toproduceacopyofthecurrent
flowingintooroutofaninput
terminalbyreplicatingthe
currentinanoutputterminal.

3. •Animportantfeatureofthecurrentmirrorisarelatively
highoutputresistancewhichhelpstokeeptheoutput
currentconstantregardlessofloadconditions.
•Anotherfeatureofthecurrentmirrorisarelativelylow
inputresistancewhichhelpstokeeptheinputcurrent
constantregardlessofdriveconditions.
•Thecurrentbeing'copied'canbe,andoftenis,avarying
signalcurrent.
•Thecurrentmirrorisoftenusedtoprovidebiascurrentsand
activeloadsinamplifierstages.

4. Theidealblocklevelconceptofthecurrentmirror,seefigure.
Givenacurrentsourceastheinput,theinputsectionofthe
currentmirrorlookslikeavirtualshortcircuitandreflects
(swapsthedirectionofflow)thiscurrenttoproduceacurrent
sink(thecurrentexitingthemirror);asaresult,weobtaina
currentsink. (I
C1=I
IN)
I
C1=(V
I–V
BE1/R);V
BE1=V
BE2THENI
OUT=I
IN
V
BE1
V
BE2

5. •Wecangeneralizethisbasiccurrentmirrorstructurewith
thisfirstobservation:Acurrentmirrorconsistsofalow
impedanceinputstageconnectedtoahighimpedance
outputcurrentstage.
•Anidealcurrentmirrorissimplyanidealcurrentamplifier
withagainof-1.
•Becauseinacurrentmirrortheinputandoutputare
currents,itiseasiertoconverttheinputtoavoltagefirstand
thenconvertavoltagebacktoacurrentattheoutput.